Re: [PATCH] sched/fair: Clear ->h_load_next after hierarchical load

[Peng Wang]

On Wed, Oct 15, 2025 at 03:14:37PM +0200, Vincent Guittot wrote:
> On Wed, 15 Oct 2025 at 14:44, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Wed, Oct 15, 2025 at 08:19:50PM +0800, Peng Wang wrote:
> >
> > > We found that the task_group corresponding to the problematic se
> > > is not in the parent task_group’s children list, indicating that
> > > h_load_next points to an invalid address. Consider the following
> > > cgroup and task hierarchy:
> > >
> > >          A
> > >         / \
> > >        /   \
> > >       B     E
> > >      / \    |
> > >     /   \   t2
> > >    C     D
> > >    |     |
> > >    t0    t1
> > >
> > > Here follows a timing sequence that may be responsible for triggering
> > > the problem:
> > >
> > > CPU X                   CPU Y                   CPU Z
> > > wakeup t0
> > > set list A->B->C
> > > traverse A->B->C
> > > t0 exits
> > > destroy C
> > >                         wakeup t2
> > >                         set list A->E           wakeup t1
> > >                                                 set list A->B->D
> > >                         traverse A->B->C
> > >                         panic
> > >
> > > CPU Z sets ->h_load_next list to A->B->D, but due to arm64 weaker memory
> > > ordering, Y may observe A->B before it sees B->D, then in this time window,
> > > it can traverse A->B->C and reach an invalid se.
> >
> > Hmm, I rather think we should ensure update_cfs_rq_h_load() is
> > serialized against unregister_fair_sched_group().
> 
> The bug has been reported for v5.10 which probably don't have fixed
> done "recently"
> commit b027789e5e50 ("sched/fair: Prevent dead task groups from
> regaining cfs_rq's")

Hi, Vincent and Peter,

We have already integrated this commit, but the bug persists.

Do you think we should explicitly clear the h_load_next list?

Even though update_cfs_rq_h_load runs under an RCU lock, ARM's
weak memory ordering could still allow readers to observe stale
values in the list.

> 
> >
> > And I'm thinking that really shouldn't be hard; note how
> > sched_unregister_group() already has an RCU grace period. So all we need
> > to ensure is that task_h_load() is called in a context that stops RCU
> > grace periods (rcu_read_lock(), preempt_disable(), local_irq_disable(),
> > local_bh_disable()).
> >
> > A very quick scan makes me think at the very least the usage in
> >
> >   task_numa_migrate()
> >     task_numa_find_cpu()
> >       task_h_load()
> >
> > fails here; probably more.